Large instruments can typically comprise several modules. These modules may be located physically apart from each other. Consequently, design of the instrument can be troublesome, particularly when all of the modules are required to have a common trigger which may be driven by any particular module and received by all of the modules. The common trigger is required so that each of the modules begin measurements at the same time. If they do not start at the same time, then errors will be introduced into the measured results.
For example, as shown in FIG. 5, large instrument 50 includes several modules 51, 52, and 53. Each module has an open collector driver 54. The trigger outputs 55 are connected together by an external connection 56, typically a coaxial cable. One module would have a pull-up resistor (not shown) to set the trigger line to the inactive or non-triggered state. Each of the modules or instruments would be able to monitor the trigger line. Note that the modules may be separated by distances of a few inches to several yards. As the number of modules increases, the shared trigger line 56 becomes longer and longer. Thus, the transmission line discontinuities grow. As each module attempts to drive the trigger line the waveform will not be seen as a clean edge transition by all other modules.
The actual waveform 61 of the trigger line of FIG. 5 is shown in FIG. 6. The desired or ideal waveform 62 is shown for comparison. Note that the fall time of trigger line waveform 61 is very slow, as compared to the ideal waveform 62. This is due to the large distributed capacitance inherent in the trigger line from the large number of modules and the length of the line to interconnect them, for example 30 modules interconnected with a total of 30 feet of cable. Thus, any module listening to the trigger line would have difficulties determining where the edge transition occurs. This difficulty is especially prevalent if digital switching noise is also present. Also note that transmission line or trigger line impedance mismatches can cause reflections. This is depicted in the oscillations or over-shoot and under-shoot in the trigger line waveform 61. Undershoot can cause receiver circuits to falsely sense a trigger event.
Furthermore, the large number of modules, and the long distances between the first module and the last module would introduce a long propagation delay into the signal. Thus, as the signal travels down the trigger line, skew is introduced between one module and the next. This skew accumulates as the signal travels to each subsequent module and could reach 100 nanoseconds or even higher. Thus, all of the modules would not start their measurements at the same time.
A prior art solution to this problem is to form a star topology, instead of a chain, wherein a first module would generate the trigger stimulus. The stimulus would then go to a MUX which would run to each of the remaining modules via equal lengths of wire. Thus, the total delay received by each module would be equal, and each of the modules would start at the same time. However, this arrangement does not allow for each module to trigger an event, as only the center or first module is connected to the MUX. Each module must be wired to the remaining modules of the instrument to permit any particular module to initiate a trigger event. This requires a great deal of interconnect, which quickly becomes untenable as the number of modules increases. For example, a 10 module system requires 90 wires, while a 20 module system requires 380 wires. Note that each of the wires must be of the same length. This is known as a fully meshed topology.
Therefore, there is a need in the prior art for a clock and trigger mechanism which will allow for each of the modules to initiate a trigger event, while still having all of the modules start their measurements at the same time. In other words, the system would have little skew, which would allow for accurate measurements.